The present invention relates to artificial kidneys which perform the four functions of the human kidney. It maintains a water balance, acid-base balance, electrolyte balance, and removes nitrogeneous wastes, such as urea and creatinine. All of these functions generally involve the transfer of relatively small molecules having a molecular weight of less than 160 daltons.
It has been found that with extended dialysis, patients who have these molecules maintained at physiologically normal levels become afflicted with neuropathy, a nervous system disorder associated with kidney failure. It is believed that there are uremotoxins having molecular weights between 300 and 2000 daltons, which may be responsible for the illnesses. This range of molecules, the so-called "middle molecules", are generally not removed in significant quantities in prior art dialyzing techniques. Previously, low flux membranes, having ultrafiltration coefficients of less than 20 ml m.sup.-2 hr.sup.-1 mmHg.sup.-1 and reflection coefficients of about zero for solutes up to a molecular weight of 300 daltons, were utilized with transmembrane pressures of 50 mmHg or greater. The relative low porosity of the low flux membrane limited the ultrafiltration (water loss from the patient's blood) to acceptable levels. The high transmembrane pressure was necessary in order to achieve a reasonable level of dialysis in the single pass systems.
One alternate approach is to utilize a closed loop dialyzing system with a high flux membrane, having an ultrafiltration coefficient of at least 20 ml m.sup.-2 hr.sup.-1 mmHg.sup.-1 and reflection coefficients of about zero for solutes up to a molecular weight of 2000 daltons. Because the dialysate has a fixed volume which is merely recirculated past the dialyzing membrane, and the dialysate is relatively incompressible, there will, of necessity, be little or no ultrafiltration across the dialyzing membrane. Unfortunately, because the fixed volume (generally 70 liters) of the dialysate is not much larger than the patient's "fluid compartment" (the volume of cellular and blood liquid which retains the uremotoxins, in a human about 40 liters), the final concentration of uremotoxins, if a complete concentration balance were obtained between the patient and the dialysate, will be relatively high. Furthermore, because a patient is on a dialysis machine for a relatively short period of time, the solutes never equilibrate and, thus even higher levels of uremotoxins remain in the patient after dialysis. Although this system has the capability for removing some "middle molecules" while controlling ultrafiltration, the system has a very low efficiency for the removal of the more conventional small molecular weight solutes.
Clearly, if a high flux membrane were utilized in a conventional open loop dialyzer, with conventional transmembrane pressures, on the order of 50 mmHg, an excessively high ultrafiltration rate would result, preventing the patient from being connected to the dialyzer for the required period of time.